<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <style>
    input {
      padding: 5px 15px;
      height: 46px;
      font-size: 18px;
    }
    input::-webkit-input-placeholder { 
      /* WebKit browsers */ 
      color: red;
      font-size: 18px;
    } 
    input:-moz-placeholder { 
      /* Mozilla Firefox 4 to 18 */ 
      color: red; 
      font-size: 18px;
    } 
    input::-moz-placeholder { 
      /* Mozilla Firefox 19+ */ 
      color: red; 
      font-size: 18px;
    } 
    input:-ms-input-placeholder { 
      /* Internet Explorer 10+ */ 
      color: red; 
      font-size: 18px;
    }
  </style>
</head>
<body style="background-color: #6b6b6b;">
  <input type="text" placeholder="这里是说明" />
</body>
</html>